Optical scanning method, optical scanner and image forming apparatus

ABSTRACT

An optical scanner can write a full-color image without occurrence of writing position differences even if a record density is switched. The optical scanner includes a black writing illuminant for optically writing an image at a plurality of record densities and a color writing illuminant for optically writing the image at a predetermined record density. The optical scanner adjusts a resist position for a full-color image with respect to the main and sub-scanning directions by switching a writing position of the black writing illuminant in accordance with a requested record density and then writes the full-color image at that writing position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Rule 1.53(b) Continuation of U.S. Ser. No.10/452,458, filed Jun. 2, 2003, now U.S. Pat. No. 6,930,700 the entirecontents of which are incorporated by reference herein.

BACKGROUND

1. Technical Field

This disclosure generally relates to an optical scanning method, anoptical scanner and an image forming apparatus, and more particularly toan optical scanning method and an optical scanner for writing latentimages by radiating optical beams on scanned surfaces of a plurality oflinearly arranged image carrying members, and an image forming apparatussuch as a copier, a printer and a plotter that can form a multi-colorimage by developing the latent images with distinct color developers andthen sequentially transferring the developed color images to atransferred member.

2. Description of the Related Art

In a conventional tandem type color image forming apparatus, opticalbeams emitted from a plurality of illuminants are radiated to fourlinearly arranged image carrying members such as photosensitive drums inorder to write latent images thereon. The latent images formed on theimage carrying members are developed to visualize the latent images byusing distinct color developers, typically, a yellow toner, a magentatoner, a cyan toner and a black toner. Then, a transferred member suchas a recorded paper is carried on a transfer belt to each transferringpart of the image carrying members, and the individual color images aresequentially superposed on the transferred member. The resulting colorimage on the transferred member is fixed, and it is possible to producea multi-color image.

In such a conventional tandem type color image forming apparatus, anoptical scanner, such as an optical writing apparatus, is prepared foreach of the image carrying members, and the optical writing apparatuswrites a latent image on the corresponding image carrying member.However, the optical writing apparatus is relatively expensive becausethe optical writing apparatus contains an optical deflector formed of apolygon mirror and a drive motor for driving the optical deflector. Forthis reason, components and assembly costs of the conventional tandemtype color image forming apparatus can be problematic, as it isnecessary to provide a plurality of optical writing apparatusescorresponding to the plurality of image carrying members. In addition,it is necessary to provide an adequate installation space in the imageforming apparatus to accommodate the optical writing apparatuses each ofwhich includes an optical deflector. As a result, it is impossible toavoid a size increase in an image forming apparatus in which it isdesired to include such optical writing apparatuses.

Furthermore, although a tandem type color image forming apparatus iscapable of forming a color image, the occasion in offices to producemonochrome manuscripts is greater than that of color manuscripts. As thetandem type color image forming apparatus is required to produce morefull-color manuscripts at higher speeds, the tandem type color imageforming apparatus has more significant problems, including thefollowing:

1. a complicated mechanism for superposing four colors,

2. a cost increase of motors and drive parts for driving photosensitivemembers,

3. a short life span of the motors and the drive parts for driving thephotosensitive members.

In order to meet such office use, conventional color image formingapparatuses are designed to achieve higher productivity in a monochromemode than in a full-color mode; that is, to operate in the monochromemode at higher line speed than in the full-color mode. Such color imageforming apparatuses can offer monochrome manuscripts at higher speedthan full-color manuscripts; that is, the color image formingapparatuses can form more images in the monochrome mode per unit of timethan in the full-color mode.

On the other hand, there is a color image forming apparatus that allowsa user to switch between a quality priority mode and a speed prioritymode. For instance, the color image forming apparatus produces an imageat a resolution of 1200 dpi in the quality priority mode and at aresolution of 600 dpi in the speed priority mode. In the qualitypriority mode, the image forming apparatus writes an image at a higherwrite density under a constraint of lower line speed so that ahigh-quality manuscript can be obtained, albeit at the cost of a sloweroperating speed. In contrast, in the speed priority mode, the imageforming apparatus writes an image at higher line speed under aconstraint of moderate image quality so that high-speed operations canbe achieved, albeit at the cost of a lower resolution image quality.

In the above-mentioned color image forming apparatus, when a user wantsto obtain more monochrome manuscripts in the speed priority mode than inthe quality priority mode, a user is allowed to select the operationmode from the quality priority mode and the speed priority mode byswitching the pixel density. In the conventional color image formingapparatus, two beams for black (BK) are prepared therein together with apitch switching mechanism, and one beam for each of yellow (Y), magenta(M) and cyan (C) is provided therein. Then, there are four modecombinations: a monochrome quality priority mode, a monochrome speedpriority mode, a color quality priority mode, and a color speed prioritymode. In the monochrome quality priority (1200 dpi) mode, the colorimage forming apparatus operates two BK beams at a pitch of 1200 dpiwith respect to the sub-scanning direction at low line speed. In themonochrome speed priority (600 dpi) mode, the color image formingapparatus operates the two BK beams at a pitch of 600 dpi with respectto the sub-scanning direction at high line speed. In the color qualitypriority (1200 dpi) mode, the color image forming apparatus operatescolor beams and one of the two BK beams, each of which writes an imageat the pitch of 1200 dpi with respect to the sub-scanning direction atlow line speed. At this time, only one of the two BK beams is switchedON. In the color speed priority (600 dpi) mode, the color image formingapparatus operates the color beams and one of the two BK beams, each ofwhich writes an image at the pitch of 600 dpi with respect to thesub-scanning direction at high line speed.

According to the above-mentioned color image forming apparatus, whenresist positions of four colors (BK, C, M, Y) are adjusted with respectto the main scanning direction and the sub-scanning direction (only onebeam is used for BK), it is necessary to properly set a pixel densityswitching position of BK as either 600 dpi or 1200 dpi. If the pixeldensity switching position is not properly adjusted, there is aprobability that a produced full-color image has a color difference dueto misalignment of the BK write position as shown in FIGS. 1A and 1B.

FIGS. 1A and 1B show dot positions of optical spots for two-beam writingunder two pixel densities of 1200 dpi and 600 dpi. FIG. 1A shows dotpositions of a first beam and a second beam at a resolution of 1200 dpi,and FIG. 1B shows dot positions of a first beam and a second beam at aresolution of 600 dpi.

As shown in FIG. 1A, when an image is written at the resolution of 1200dpi with respect to the sub-scanning direction, a pitch of 21 μm (=25.4mm/1200) between adjacent optical spots is obtained. As shown in FIG.1B, when an image is written at the resolution of 600 dpi with respectto the sub-scanning direction, a pitch of 42 μm (=25.4 mm/600) betweenadjacent optical spots is obtained. As seen in FIGS. 1A and 1B, a dotposition of an optical spot has a difference L of 10.5 μm between thetwo resolutions, as computed from the following formula:L=(42 μm−21 μm)/2.

FIG. 2 shows the difference of dot positions with respect to thesub-scanning direction between the two resolutions. When one of the BKbeams is used in the full color modes, there is a probability that acolor difference between BK and another color (cyan in FIG. 2) may occurwith respect to the sub-scanning direction if beam pitch positions arenot properly adjusted for alternation between the two resolutions of1200 dpi and 600 dpi. This color difference is caused by the narrowedbeam pitch between BK and the other color by the difference L.

On the other hand, FIGS. 3A through 3C show a difference of dotpositions with respect to the main scanning direction between the tworesolutions. As shown in FIG. 3A, full-color adjustment for properlyproducing full-color images is performed for the first beam with respectto the main scanning direction. In fact, however, if the second beam,which is not adjusted, is used to form the full-color images, a colordifference arises between the second beam and the other color beams withrespect to the main scanning direction, as is shown in FIG. 3B. Aspreviously mentioned, this color difference is caused by the differenceof dot positions of BK beams as shown in FIG. 3C. As used herein, thephrase “full-color adjustment” means to correct color differences causedat shipment and during use. Japanese Laid-Open Patent Application No.11-301032 discloses an adjustment technique for correcting such colordifferences.

SUMMARY

In an aspect of this disclosure, there is provided an optical scannerthat has a write density switching function to correct misalignment of awriting position of a full-color image even if a BK write density isswitched.

In an aspect of this disclosure, there is provided an image formingapparatus that can form a full-color image without any color differenceeven if the BK write density is switched.

In an exemplary embodiment of this disclosure there is provided anoptical scanning method for writing an image in an image formed mediumby using a black writing illuminant and a color writing illuminantwherein the black writing illuminant writes the image at a plurality ofrecord densities and the color writing illuminant writes the image at apredetermined record density, the optical scanning method including thesteps of: adjusting a resist position for a full-color image withrespect to a main scanning direction and a sub-scanning direction bychanging a writing position of the black writing illuminant inaccordance with a requested one of the record densities; and writing thefull-color image at the writing position in the image formed medium.

In the above-mentioned embodiment, when the record density or writingspeed is changed at formation time of a full-color image, it is possibleto write the full-color image at a writing position suitable to thefull-color image formation. As a result, there is no probability that acolor difference arises due to misalignment of the writing position.

In another exemplary embodiment of this disclosure, there is provided anoptical scanner for writing an image in an image formed medium,including: a black writing illuminant optically writing the image at aplurality of record densities; a color writing illuminant opticallywriting the image at a predetermined record density; a storage partstoring writing position data of the black writing illuminantcorresponding to the record densities; and a writing position switchingpart switching a writing position of the black writing illuminant basedon the writing position data in the storage part so as to properly forma full-color image, wherein the writing position data are used to adjusta resist position for the full-color image with respect to a mainscanning position and a sub-scanning position.

In the above-mentioned embodiment, when a full-color image is written,it is possible to properly write the full-color image by switching awriting position of the black writing illuminant into a state wherecolor differences due to shipment and use thereof are corrected. As aresult, even if the record density and the writing speed havedifferences from those in the corrected state, there is no probabilitythat a color difference arises due to misalignment of the writingposition.

In the above-mentioned optical scanner, the black writing illuminant mayinclude at least two semiconductor lasers, a retaining part retainingthe semiconductor lasers in a state where the semiconductor lasers arefixed relative to each other, a supporting part supporting the retainingpart such that the retaining part can be rotated with respect to apredetermined rotational axis, and a driving part rotating the retainingpart with respect to the rotational axis.

According to the above-mentioned embodiment, even if the black writingilluminant is constituted as a two-beam illuminant, it is possible toeasily adjust writing positions of two beams from the black writingilluminant by simply setting a rotational position thereof.

In the above-mentioned optical scanner, the driving part may include astepping motor.

According to the above-mentioned embodiment, since a rotation angle ofthe black writing illuminant can be determined through the number ofsteps of the stepping motor, it is possible to easily control therotation angle.

In the above-mentioned optical scanner, the writing position switchingpart may drive the stepping motor so as to switch the writing positionof the black writing illuminant based on the writing position data inthe storage part.

According to the above-mentioned embodiment, when a full-color image isformed, it is possible to automatically switch record densities by usingthe writing position switching part.

In the above-mentioned optical scanner, the black writing illuminant mayhave two semiconductor lasers, and the rotational axis may be located atone of a middle point between writing positions of the two semiconductorlasers and a writing position of one of the two semiconductor lasers.

According to the above-mentioned embodiment, it is possible to determinethe writing position through the rotation angle. If a relationshipbetween writing positions and rotation angles is prescribed in advance,it is possible to easily set a desired position as the writing position.

In another aspect of this disclosure, there is provided an image formingapparatus, including: an optical scanner writing an image in an imageformed medium, the optical scanner comprising: a black writingilluminant optically writing the image at a plurality of recorddensities; a color writing illuminant optically writing the image at apredetermined record density; a storage part storing writing positiondata of the black writing illuminant corresponding to the recorddensities; and a writing position switching part switching a writingposition of the black writing illuminant based on the writing positiondata in the storage part so as to properly form a full-color image,wherein the writing position data are used to adjust a resist positionfor the full-color image with respect to a main scanning position and asub-scanning position; and an image forming part developing individualcolor images written by the optical scanner and forming the full-colorimage on a record medium.

According to the above-mentioned apparatus, it is possible to properlyform the full-color image written by the optical scanner without anycolor difference.

In the above-mentioned image forming apparatus, the optical scanneroptically may write the individual color images on image carryingmembers, which are linearly arranged, corresponding to the color images.

According to the above-mentioned aspect, when the above-mentioned tandemtype image forming apparatus is used to write a full-color image inlinearly arranged image carrying members corresponding to individualcolors, it is possible to suppress color differences. The tandem typeimage forming apparatus includes a plurality of illuminant units of amulti-beam black writing illuminant having the record density switchingpart and single-beam color writing illuminants. When the tandem typeimage forming apparatus writes latent color images by irradiatingoptical beams on scanned surfaces of the image carrying members therein,the tandem type image forming apparatus changes a writing position ofthe black writing illuminant by adjusting resist positions of the blackimage with respect to the main scanning direction and the sub-scanningdirection (regardless of record densities of 600 dpi, 1200 dpi and 2400dpi). Then, the tandem type image forming apparatus writes the blackimage at that writing position. As a result, there is no probability ofwriting position differences occurring. Furthermore, users can select aquality priority mode and a speed priority mode by switching the recorddensities so that the image forming apparatus can produce moremonochrome images than full-color images.

Other aspects, features and advantages will become more apparent fromthe following detailed description when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams illustrating dot positions of optical spotsfor two-beam writing under two pixel densities of 1200 dpi and 600 dpi,respectively;

FIG. 2 is a diagram illustrating a difference of dot positions withrespect to a sub-scanning direction between the two pixel densities;

FIGS. 3A through 3C are diagrams illustrating a difference of dotpositions with respect to the main scanning direction between the twopixel densities;

FIG. 4 is a side elevational view roughly illustrating a structure of animage forming apparatus according to the present invention;

FIG. 5 is a top plan view of an optical scanner according to the presentinvention;

FIG. 6 is a diagram illustrating arrangement of an optical deflector andoptical systems in the optical scanner according to the presentinvention;

FIG. 7 is a cross-sectional view of the optical scanner according to thepresent invention as viewed from the plane A-A′ in FIG. 5;

FIG. 8 is a diagram illustrating arrangement of illuminant units, theoptical deflector and the optical systems in the optical scanneraccording to the present invention;

FIG. 9 is an exploded perspective view of a multi-beam illuminant unitof the optical scanner according to the present invention;

FIG. 10 is a cross-sectional view of the multi-beam illuminant unit ofthe optical scanner according to the present invention;

FIG. 11 is a diagram for explaining rotation adjustment of themulti-beam illuminant unit of the optical scanner according to thepresent invention;

FIGS. 12A and 12B are diagrams illustrating shift positions of opticalspots on a photosensitive drum corresponding to rotation angles of themulti-beam illuminant unit of the optical scanner according to thepresent invention; and

FIG. 13 is a flowchart of a procedure for adjusting a pitch betweenoptical spots from the multi-beam illuminant unit of the optical scanneraccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be describedwith reference to the accompanying drawings.

A description will now be given, with reference to FIG. 4, of an imageforming apparatus according to the present invention.

FIG. 4 roughly shows a structure of the image forming apparatusaccording to the present invention. The image forming apparatuscomprises a plurality of drum-shaped photoconductive photosensitivemembers (which are referred to as photosensitive drums hereinafter) 1,2, 3 and 4, electrifying parts 6, 7, 8 and 9, an optical scanner 5serving as an exposing part, developing parts 10, 11, 12 and 13, atransferring-carrying apparatus 22, cleaning parts 18, 19, 20, and 21.As shown in FIG. 4, the image forming apparatus is a full-color imageforming apparatus in that the photosensitive drums 1, 2, 3 and 4, whichare linearly arranged therein, are used to form color imagescorresponding to individual colors such as black (BK), cyan (C), magenta(M) and yellow (Y), respectively. The photosensitive drums 1, 2, 3 and 4are not limited to the as-shown arrangement and may be arranged in anysuitable manner. As shown in FIG. 4, the other above-mentioned parts(the electrifying parts 6, 7, 8 and 9, the developing parts 10, 11, 12and 13, and the cleaning parts 18, 19, 20 and 21) for forming imagesthrough electrophotographic processing are provided around therespective photosensitive drums 1, 2, 3 and 4. The electrifying parts 6,7, 8 and 9 are formed of charge rollers, charge brushes, electrifyingcharger, for example. The optical scanner 5, which is to be described ingreater detail below, uses optical beams L1, L2, L3 and L4 to exposescanned surfaces of the photosensitive drums 1, 2, 3 and 4. Thedeveloping parts 10, 11, 12 and 13 serve as developing apparatuses, eachof which corresponds to individual colors of BK, C, M and Y. Thetransferring-carrying apparatus 22 includes a transferring-carrying belt22 a and transferring parts 14, 15, 16 and 17, which are formed oftransferring rollers and transferring brushes, for example, in theinward-facing side of the transferring-carrying belt 22 a. The cleaningparts 18, 19, 20 and 21 are formed of cleaning blades and cleaningbrushes, for example. In this configuration, the image forming apparatusaccording to the present invention can form individual color images onthe photosensitive drums 1, 2, 3 and 4.

In FIG. 4, the X and Y directions represent horizontal directions of aspace where the image forming apparatus is located, and the Z directionrepresents a vertical direction thereof. As shown in FIG. 4, the fourphotosensitive drums 1, 2, 3 and 4 are linearly arranged to have a slopewith respect to the X-Y plane. In FIG. 4, the photosensitive drums 1, 2,3 and 4 are arranged to have a negative slope in the Z-X coordinatedirections. The transferring-carrying apparatus 22 is providedslantingly relatively to the X-Y plane in nearly parallel relation tothe arrangement of the four photosensitive drums 1, 2, 3 and 4. Atransferred member such as a record paper is fed from the lower end ofthe sloped arrangement of parts and is carried upwards to transferringparts 14, 15, 16 and 17 of the photosensitive drums 1, 2, 3 and 4sequentially on the transferring-carrying belt 22 a. A fixing apparatus26 is provided at upper end of the sloped arrangement of parts, that is,the lower stream of the carrying direction of the transferred member.Also, the optical scanner 5 is mounted around an upper corner of thelinearly arranged photosensitive drums 1, 2, 3 and 4, which serve asimage forming parts. A housing 50 of the optical scanner 5 is mountedslantingly relative to the X-Y plane such that the housing 50 is nearlyparallel to the arrangement of the photosensitive drums 1, 2, 3 and 4.The housing 50 is fixed to sloped frame members 29 and 30 of the imageforming apparatus.

FIG. 5 is a top plan view of the optical scanner 5 from the upperportion thereof. FIG. 6 shows an arrangement of an optical deflector andoptical systems in the optical scanner 5.

Referring to FIG. 5 and FIG. 6, the optical scanner 5 comprises fourilluminant units 52, 53, 54 and 55, an optical deflector 62, beamfocusing lenses 63, 64, 69, 70, 71 and 72, optical path folding mirrors65, 66, 67, 68, 73, 74, 75, 76, 77, 78, 79 and 80. These components areaccommodated in the housing 50. The four illuminant units 52, 53, 54 and55 emit optical beams L1, L2, L3 and L4, respectively. The opticaldeflector 62 deflects the optical beams L1, L2, L3 and L4 such that twopairs of the four beams L1, L2, L3 and L4 propagate in two directionssymmetric to each other. Image forming optical systems, which includethe beam focusing lenses 63, 64, 69, 70, 71 and 72, and the optical pathfolding mirrors 65, 66, 67, 68, 73, 74, 75, 76, 77, 78, 79 and 80, leadthe deflected optical beams L1, L2, L3 and L4 on scanned surfaces of thecorresponding photosensitive drums 1, 2, 3 and 4, as illustrated in FIG.6.

FIG. 7 is a cross-sectional view of the optical scanner 5 as viewed fromthe A-A′ plane in FIG. 5.

As shown in FIG. 5 and FIG. 7, the housing 50 includes a substrate 50Ato which the optical deflector 62 and the optical systems are mounted,and a sidewall 50B for surrounding the substrate 50A. The substrate 50Ais located near the center of the housing 50 with respect to thevertical direction thereof (top to bottom in FIG. 7) and partitions aninner space in the housing 50 into upper and lower portions. The fourilluminant units 52, 53, 54 and 55 are mounted to the sidewall 50B (FIG.5) in almost parallel relation to the arrangement of the photosensitivedrums 1, 2, 3 and 4 (FIG. 4). As shown in FIG. 7, the optical deflector62 is located in the center of the substrate 50A. The above-mentionedoptical systems such as the beam focusing lenses 63, 64, 69, 70, 71 and72 and the optical path folding mirrors 65, 66, 67, 68, 73, 74, 75, 76,77, 78, 79 and 80 are provided in both portions (the upper portion andthe lower portion) of the inner space partitioned by the substrate 50A.Also, covers 87 and 88 are provided in the lower side and the upper sideof the housing 50, respectively. The lower cover 87 has apertures forpassage of optical beams, and dustproof glasses 83, 84, 85 and 86 areprovided to each of the apertures.

FIG. 8 shows an arrangement of the illuminant units 52, 53, 54 and 55,the optical deflector 62 and the optical systems in the optical scanner5. As shown in FIG. 8, when image data of individual colors are providedto the optical scanner 5 from a manuscript reading apparatus such as ascanner or an image data output apparatus such as a personal computer, aword processor, or a facsimile receiver, which are not illustrated, theoptical scanner 5 converts the individual color image data into signalsfor driving the illuminant units 52, 53, 54 and 55. The illuminant units52, 53, 54 and 55 emit optical beams in accordance with the signals. Theoptical beams propagate through cylindrical lenses 56, 67, 58 and 59 forcorrecting optical face tangle errors and then arrive at the opticaldeflector 62 directly or via mirrors 60 and 61. The optical beams aredeflected in the two symmetric directions by two-tiered polygon mirrors62 a and 62 b rotated by a polygon motor 62 c at constant speed as shownin FIGS. 7 and 8. Here, the two-tiered polygon mirrors 62 a and 62 bdeflect a pair of the optical beams L2 and L3 and a pair of the opticalbeams L1 and L4, respectively. Although the optical deflector 62, asillustrated, uses two polygon mirrors 62 a and 62 b, the opticaldeflector 62 may use, for example, one large polygon mirror to deflectthe four optical beams L1, L2, L3 and L4.

As shown in FIG. 7, the deflected optical beams are transmitted throughimage forming lenses 63 and 64. For instance, two-tiered fθ lenses canbe used as the image forming lenses 63 and 64. The optical beams L1, L2,L3 and L4 are folded by the first folding mirrors 65, 66, 67 and 68 andthen travel through the apertures of the substrate 50A. After passagethrough the apertures of the substrate 50A, the optical beams L1, L2, L3and L4 travel through the second image forming lenses 69, 70, 71 and 72,and then arrive on the scanned surfaces of the photosensitive drums 1,2, 3 and 4 via the second folding mirrors 73, 75, 77 and 79, the thirdfolding mirrors 74, 76, 78 and 80, and the dustproof glasses 83, 84, 85and 86. When the optical beams L1, L2, L3 and L4 are irradiated on thescanned surfaces of the photosensitive drums 1, 2, 3 and 4, it ispossible to write latent images on the scanned surfaces.

In the optical scanner 5, each of the illuminant units 52, 53, 54 and 55comprises a semiconductor laser (LD) working as an illuminant and acollimate lens for collimating a luminous flux emitted by thesemiconductor laser. The semiconductor laser and the collimate lens areintegrally accommodated in a retaining member such as a holder. In theillustrated embodiment, the illuminant unit 52 is a BK illuminant unit.Since the BK illuminant unit 52 is more frequently used than any othercolor illuminant units to form monochrome images, it is preferable thatthe illuminant unit 52 be constituted as a multi-beam illuminant unitwherein at least two pairs of illuminants and collimate lenses areintegrally accommodated in the retaining member thereof. As a result,when the monochrome images are formed, it is possible to optically writethe monochrome images at a high speed and, therefore, to improveproductivity of the image forming apparatus with respect to monochromeimage formation.

A description will now be given, with reference to FIG. 9 and FIG. 10,of a multi-beam illuminant unit serving as a BK illuminant unit. FIG. 9is an exploded perspective view of the multi-beam illuminant unit. FIG.10 is a cross-sectional view of the multi-beam illuminant unit.

Referring to FIG. 9 and FIG. 10, semiconductor lasers 111 and 112, whichserve as illuminants of the multi-beam illuminant unit 52, are fixed tosupporting members 113 and 114, respectively. The semiconductor lasers111 and 112 are connected to a collimate lens holder 115 with fasteners118 and 119 via the supporting members 113 and 114 such that opticalbeams from the semiconductor lasers 111 and 112 coincide with opticalaxes of collimate lenses 116 and 117, respectively. The collimate lenses116 and 117 are accommodated in cylindrical mirror holders and areconnected to holes 115 a and 115 b in the collimate lens holder 115 by asuitable adhesive such that the collimate lenses 116 and 117 arepositioned, as illustrated, for example, relative to the respectivesemiconductor lasers 111 and 112. The collimate lenses 116 and 117convert the optical beams from the semiconductor lasers 111 and 112 intoparallel luminous fluxes. An iris plate 120 is provided at the exit endof the collimate lenses 116 and 117 so that each of the outgoing opticalbeams can have a predetermined beam diameter. A beam synthesizing part121 such as a prism is provided behind or downstream of the iris plate120.

The two semiconductor lasers 111 and 112 are arranged in the same planesuch that a pn junction surface of the semiconductor laser 111 coincideswith that of the semiconductor laser 112. A ½ wavelength plate 122 isprovided at the entrance end of the beam synthesizing part 121 so as torotate by 90° a polarization surface of one of the two optical beamsfrom the semiconductor lasers 111 and 112, for example, the optical beamfrom the semiconductor laser 111 in the illustrated embodiment. Theresulting optical beam whose polarization surface is rotated by 90°travels to a polarization beam splitter surface 121 b (FIG. 9) of thebeam synthesizing part 121. The optical beam from the semiconductorlaser 112, on the other hand, is inner-reflected on a sloped surface 121a of the beam synthesizing part 121 and also is reflected on thepolarization beam splitter surface 121 b. The resulting optical beamfrom the semiconductor laser 112 is synthesized with the optical beamfrom the semiconductor laser 111 in the vicinity of the optical axis ofthe optical beam from the semiconductor laser 111, which is consideredas a reference optical beam. At this time, the optical axes of thesemiconductor lasers 111 and 112 are directed in slightly differentdirections from each other with respect to the main scanning direction.Here, an angle between the optical axes is set as θ in the exit side ofthe beam synthesizing part 121 as shown in FIG. 9.

The beam synthesizing part 121 and the iris plate 120 are mounted atpredetermined positions on an entrance or upstream surface of a flangemember 123, and the flange member 123 is fixed to the collimate lensholder 115 with fasteners 124 and 125. The flange member 123 and/or thecollimate lens holder 115 is fixed on (not illustrated) a substrate 126on which a drive circuit for driving the semiconductor lasers 111 and112 is provided. In this configuration, the members along the opticalpaths between the semiconductor lasers 111 and 112 and the flange member123 are fixed on the substrate 126, and these members constitute theilluminant unit 52.

As shown in FIG. 9, a cylinder part 123 a is mounted to the exit ordownstream end of the flange member 123. The cylinder part 123 a isinserted into a hole 132 a of a frame 132 provided on the sidewall 50Bof the housing 50 (FIG. 7). The cylinder part 123 a is inserted throughthe interior of a helical spring 130 and further through a hole 131 a ofa spring pressure plate 131. In this configuration, if the BK illuminantunit 52, which has the members between the semiconductor lasers 111 and112 and the flange 123 on the substrate 126, is pulled in the directionof the arrow α in FIG. 9 and then the spring pressure plate 131 isrotated 90°, it is possible to hook a projection 131 b of the springpressure plate 131 on a projection 123 b of the cylinder part 123 a. Asa result, the BK illuminant unit 52 is mounted to the frame 132 in astate where the BK illuminant unit 52 can be freely rotated with respectto the center (optical axis) of the cylinder part 123 a of the flangemember 123.

Since the illuminant unit 52 can be rotated with respect to the opticalaxis, it is possible to adjust a pitch between optical spots on thephotosensitive drum. A pitch changing part, which is describedhereinafter, is used to adjust the optical spot pitch.

In FIG. 9, a male screw whose nominal diameter is M3 is shaped on a feedscrew 128, and a female screw is shaped in the interior of a movingmember 127. The moving member 127 has a somewhat D-shaped outer body.The male screw of the feed screw 128 is inserted into the female screwof the moving member 127. The moving member 127 is inserted into aD-shaped hole of a cylinder 132 b that is provided in the frame 132 inthe housing 50, as illustrated. The moving member 127 is slidablymovable in the cylinder 132 b. Here, a rotation shaft 129 a of a pitchchange stepping motor 129 is inserted through the hole of the cylinder132 b of the frame 132. The lower end of the feed screw 128 is fixed tothe top end of the rotation shaft 129 a, for example, by means of apressure fit. The pitch change stepping motor 129 is connected to theframe 132 so that the feed screw 128, which is pressed to the rotationshaft 129 a, can be rotated through rotation of the pitch changestepping motor 129. Since the cylinder 132 b has the correspondingD-shaped hole, the moving member 127 is able to make up-and-down motions(FIG. 9).

The flange member 123 has an arm 123 c. The arm 123 c extends toward themoving member 127, and the end of the arm 123 c is in contact with thetop end of the moving member 127. A tension spring 135 is providedbetween the arm 123 c and the frame 132. The tension spring 135 pullsdown the arm 123 c so that the end of the arm 123 c can be depressed onthe moving member 127. As a result, when the moving member 127 moves inthe vertical direction through rotational motions by the pitch changestepping motor 129, the arm 123 c moves up-and-down in the verticaldirection. According to such an arrangement, the illuminant unit 52 maybe rotated wherein the center of the cylinder 123 a of the flange member123 is the rotational axis.

An optical home position sensor 133 for controlling a rotation angle ofthe illuminant unit 52 is fixed with fasteners that are not illustrated.The optical home position sensor 133 has an illuminant part 133 a and areceiver part 133 b in the side of the frame 132. A filler 123 d isprovided in the side opposite to the arm 123 c of the flange member 123.The filler 123 d has an edge part for screening between the illuminantpart 133 a and the receiver part 133 b of the home position sensor 133.A home position (HP) of the optical home position sensor is determinedas the position at the time when the edge part 123 e screens between theilluminant part 133 a and the receiver part 133 b. The home position isused as a reference position for adjusting rotation of the illuminantunit 52.

FIG. 11 is a diagram for explaining the rotation adjustment of theilluminant unit 52 for the purpose of changing a pitch of optical spotswith respect to the sub-scanning direction. In FIG. 11, dotted linesindicate positions of the illuminant part 133 a and the receiver part133 b. As mentioned above, the home position is set as the position ofthe home position sensor 133 at the screening time of the edge part 123e. The position B in FIG. 11 is a position of the home position sensor133 where the illuminant unit 52 is rotated by a rotation angle of θ1from the home position with respect to a rotational axis as the opticalaxis thereof. In order to rotate the illuminant unit 52 by the rotationangle of θ1, it is necessary to shift the moving member 127 at apredetermined distance in the upper direction by rotating the pitchchange stepping motor 129 by a predetermined number of pulses.Similarly, the position A in FIG. 11 indicates a position of the homeposition sensor 133 where the illuminant unit 52 is rotated by arotation angle of θ2 from the home position with respect to therotational axis.

FIGS. 12A and 12B show positions of optical spots on a photosensitivedrum when the illuminant unit 52 is rotated such that the home positionsensor 133 is located at the home position and the position A and B.FIG. 12A is a diagram illustrating a case where a position of one ofoptical beams from the two semiconductor lasers 111 and 112 is set asthe rotational axis, and FIG. 12B is a diagram illustrating a case wherea middle position between the two optical beams is set as the rotationalaxis. In FIGS. 12A and 12B, the lengths P1 and P2 represent pitches ofthe optical spots on the photosensitive drum with respect to thesub-scanning direction corresponding to the rotation angles of θ1 andθ2. As seen in FIGS. 12A and 12B, if the illuminant unit 52 is rotatedfrom the home position, it is possible to change the pitches of theoptical spots on the photosensitive drum and easily control the rotationby adjusting the driving number of pulses of the pitch change steppingmotor 129.

Normally, the pitches of optical spots on the photosensitive drum withrespect to the sub-scanning direction are changed in accordance with arecord density. For instance, it may be assumed that the driving pulsenumber Pa corresponding to the record density of 600 dpi is set as 42 μmand the driving pulse number Pb corresponding to the record density of1200 dpi is set as 21 μm. If the pulse numbers Pa and Pb are stored in amemory in a control part of an image forming apparatus, it is possibleto easily switch the pitches of optical spots on the photosensitive drumwith respect to the sub-scanning direction by rotating the pitch changestepping motor 129 based on the stored data regarding the driving pulsenumbers Pa and Pb in accordance with a requested record density.

Once the image forming apparatus is switched ON, the image formingapparatus locates the illuminant unit 52 at a predetermined position,for instance, by rotating the illuminant unit 52 by the rotation angle(to position B) corresponding to the record density of 600 dpi. In orderto locate the illuminant apparatus 52 at that position, when the imageforming apparatus is switched ON, the image forming apparatus returnsthe illuminant unit 52 to the home position. Thereafter, the pitchchange stepping motor 129 is driven by the pulse number Pa in apredetermined direction so as to locate the pitch change stepping motor129 at the position B. As a result, it is possible to rotate theilluminant unit 52 by the rotation angle of θ1 so that optical spots onthe photosensitive drum can have the pitch P1 corresponding to theposition B with respect to the sub-scanning direction. Here, the imageforming apparatus has information regarding the predetermined rotationangles in the memory of the control part such as a CPU (CentralProcessing Unit). Accordingly, when the record density of 1200 dpi isrequested, the image forming apparatus drives the pitch change steppingmotor 129 by the pulse number of (Pb-Pa) so that the illuminant unit 52can be rotated from the position B, which is the position correspondingto the record density of 600 dpi. As a result, it is possible toproperly change the pitch of the optical spots by rotating theilluminant unit 52 to the position A, which is the positioncorresponding to the record density of 1200 dpi.

FIG. 13 is a flowchart of the above-mentioned procedure. As shown inFIG. 13, a user selects the record density of 1200 dpi in a black mode(BK 1200 dpi mode) at step S1. At step S2, it is determined whether ornot a pitch position of a BK beam is located at the position A. At stepS3, if the pitch position is currently located at the position A, theimage forming apparatus receives print data and then performs a normalprinting process by rotating the polygon motor 62 c. On the other hand,if the pitch position is not located at the position A, the pitchposition is shifted to the position A at step S4. After the pitchposition has been shifted to the position A by rotating the pitch changestepping motor 129 by the pulse number of (Pb-Pa) at step S6, the imageforming apparatus is ready to write the sent print data at the positionA corresponding to the record density of 1200 dpi. Then, the imageforming apparatus proceeds to the step S3 to perform the writingprocedure. Here, it is noted that the pitch position is located at theposition A just after step S3.

Subsequently, if the user selects a color mode print under the recorddensity of 600 dpi, the image forming apparatus proceeds to step S8. Atstep S9, it is determined whether or not the pitch position is locatedat the position B. However, since the pitch position is located at theposition A after step S3, the branch condition at step S9 is normallydetermined as NO. Then, the pitch change stepping motor 129 is driven bythe pulse number of (Pb-Pa) in the direction opposite to the rotationaldirection at step S5 so as to move the pitch position to the position Bat step S11 and step S12. After shifting the pitch position to theposition B at step S13, the image forming apparatus receives write dataand then performs the normal printing process by rotating the polygonmotor 62 c at step S10. In this fashion, the whole procedure iscompleted at step S14.

Here, the above-mentioned procedure is automatically performed by aprinter driver of the image forming apparatus in accordance with user'srequests, received data or received instructions. Then, an image isprinted out at a requested record density.

The optical scanner 5 shown in FIG. 5 and FIG. 6 has synchronizationdetecting mirrors, which are not illustrated, for retrieving luminousfluxes of start scanning positions in the main scanning direction onoptical paths of the optical beams L1, L2, L3 and L4. Synchronizationdetectors 81 and 82 receive the luminous fluxes reflected on thesynchronization detecting mirrors and supply synchronization signals forstart timings of scanning. Furthermore, stepping motors 92, 93 and 94for adjusting skew are provided in the third folding mirrors 74, 75 and76 on the optical paths of the optical beams L1, L2 and L3,respectively, as shown in FIG. 6. The stepping motors 92, 93 and 94 areused to correct misalignment of scanning lines of the optical beams L1,L2 and L3 with reference to the scanning line of the optical beam L1.Here, the main scanning direction is defined as a direction where theoptical beams deflected by the optical deflector 62 scan thephotosensitive drums, that is, the axis directions of the photosensitivedrums. Also, the sub-scanning direction is a direction perpendicular tothe main scanning direction, that is, the rotation direction of thephotosensitive drums (moving directions of the surfaces of thephotosensitive drums). Also, the sub-scanning direction is a carryingdirection of a transferring-carrying belt 22 a to be mentioned later.For this reason, it is concluded that the width direction of thetransferring-carrying belt 22 a is the main scanning direction, and thecarrying direction thereof is the sub-scanning direction.

As shown in FIG. 4, the transferring-carrying belt 22 a is disposedunder the four photosensitive drums 1, 2, 3 and 4. Thetransferring-carrying belt 22 a is supported by drive rollers anddependent rollers and is carried in the arrow direction in FIG. 4 by thedrive rollers. Furthermore, paper feeding parts 23 and 24 foraccommodating transferred members such as record papers are provided inthe lower part of the image forming apparatus. The transferred membersin the paper feeding parts 23 and 24 are fed to thetransferring-carrying belt 22 a via paper feeding rollers, carryingrollers and a resist roller 25 and then are carried by thetransferring-carrying belt 22 a.

After the optical scanner 5 forms latent images for the individualphotosensitive drums 1, 2, 3 and 4, the latent images are developed withindividual color toners of BK, C, M and Y by the developing parts 10,11, 12 and 13. The developed toner images of individual colors aresequentially superposed on a transferred member on thetransferring-carrying belt 22 a by the transferring parts 14, 15, 16 and17 of the transferring-carrying apparatus 22. Then, the transferredmember on which the four color images are transferred is delivered tothe fixing apparatus 26 and then is fixed therein. Thereafter, theresulting transferred member is delivered to the output tray 28 by thepaper output roller 27. Here, if the image forming apparatus is in themonochrome image forming mode, the above-mentioned process is performedfor only the BK photosensitive drum 1.

According to the above-mentioned image forming apparatus, when resistadjustment is performed for a full-color image with respect to the mainscanning direction and the sub-scanning direction regardless of theresolutions of 600 dpi and 1200 dpi, the image forming apparatus canadjust the BK pixel density position at a predetermined position andwrite the full-color image at the adjusted pixel density position. As aresult, it is possible to provide the tandem type color image formingapparatus that can overcome misalignment of writing positions of thecolor image.

Here, the above-mentioned embodiments concentrate on the optical scannerand the image forming apparatus that can switch the write density intothe two resolutions of 600 dpi and 1200 dpi. However, the opticalscanner and the image forming apparatus according to the presentinvention can also deal with a resolution of 2400 dpi in addition to theresolutions of 600 dpi and 1200 dpi in a similar configuration.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority application No.2002-169989 filed Jun. 11, 2002, the entire contents of which are herebyincorporated by reference.

1. An optical scanning method for writing an image in an image carryingmember by using a black writing illuminant and a color writingilluminant wherein said black writing illuminant writes said image at aplurality of record densities and said color writing illuminant writessaid image at a single predetermined record density, the methodcomprising the steps of: adjusting a beam pitch position for afull-color image with respect to a main scanning direction and asub-scanning direction by changing a beam pitch position of said blackwriting illuminant in accordance with one of the plurality of recorddensities such that the beam pitch among black and the other colors tobe equal interval; determining whether the beam pitch position of saidblack writing illuminant is located at said adjusted beam pitch positionwhen writing said full-color image; switching the beam pitch position ofsaid black writing illuminant in accordance with said adjusted beampitch position when the beam pitch position of said black writingilluminant is determined to be different from said adjusted beam pitchposition; and writing said full-color image at said adjusted beam pitchposition in said image carrying member.
 2. An optical scanner forwriting an image in an image carrying member, comprising: a blackwriting illuminant for optically writing said image at a plurality ofrecord densities; a color writing illuminant for optically writing saidimage at a single predetermined record density; a storage part forstoring data of beam pitch position of said black writing illuminantcorresponding to said plurality of record densities; and a beam pitchposition switching part for switching a beam pitch position of the blackwriting illuminant based on said data of beam pitch position in thestorage part so as to properly form a full-color image when the beampitch position of said black writing illuminant is different from saidbeam pitch position stored in said storage part, the beam pitch positionbeing adjusted in accordance with one of the plurality of recorddensities of said black writing illuminant such that the beam pitchamong black and the other colors to he equal interval.
 3. The opticalscanner as claimed in claim 2, wherein said black writing illuminantincludes at least two semiconductor lasers; a holding part for holdingsaid semiconductor lasers such that said semiconductor lasers are fixedrelative to each other; a supporting part for supporting said holdingpart such that said holding part can be rotated with respect to apredetermined rotational axis; and a driving part for rotating saidholding part with respect to said rotational axis.
 4. The opticalscanner as claimed in claim 3, wherein said black writing illuminant hastwo semiconductor lasers, and said rotational axis is located at amiddle point between said beam pitch positions of said two semiconductorlasers or one of said beam pitch positions of the two semiconductorlasers.
 5. The optical scanner as claimed in claim 3, wherein saiddriving part comprises a stepping motor.
 6. The optical scanner asclaimed in claim 5, wherein said black writing illuminant has twosemiconductor lasers, and said rotational axis is located at a middlepoint between said beam pitch positions of said of said twosemiconductor lasers or one of said beam pitch positions or the twosemiconductor lasers.
 7. The optical scanner as claimed in claim 5,wherein said beam pitch position switching part drives said steppingmotor so as to switch the beam pitch position or the black writingilluminant based on the data of beam pitch position stored in thestorage part.
 8. The optical scanner as claimed in claim 7, wherein saidblack writing illuminant has two semiconductor lasers, and saidrotational axis is located at a middle point between said beam pitchpositions of said two semiconductor lasers or one of said beam pitchpositions of the two semiconductor lasers.
 9. An image formingapparatus, comprising: an optical scanner for writing an image in animage carrying member; and an image farming pan for developingindividual color images written by said optical scanner and forming afull-color image on a record medium, wherein said optical scannercomprises a black writing illuminant for optically writing said image ata plurality of record densities; a color writing illuminant foroptically writing said image at a single predetermined record density; astorage part for storing data of beam pitch position of said blackwriting illuminant corresponding to said plurality of record densities;and a beam pitch position switching pan for switching a beam pitchposition of the black writing illuminant based on said data of beampitch position in the storage part so as to properly form a full-colorimage when the beam pitch position of said black writing illuminant isdifferent from said beam pitch position stored in said storage part, thebeam pitch position being adjusted in accordance with one of theplurality of record densities of said black writing illuminant such thatthe beam pitch among black and the other colors to be equal interval.10. The image forming apparatus as claimed in claim 9, wherein saidoptical scanner optically writes the individual color images on each ofcorresponding image carrying members linearly arranged to correspond tothe type of the colors written by said optical scanner.